Damage was sustained to the Icelander's fibula but due to the amount of swelling, X-rays taken at a hospital in Sheffield failed to provide any conclusive evidence.
Aron will have a second X-ray to assess the damage which could be a fracture this week.
If a fracture is confirmed, City will be without Gunnarsson for up to eight weeks and City boss Chris Coleman, who is already without Sammy Clingan has started the hunt for a loanee midfielder.